You’re hanging out after the ride and someone says, “Man, that hill was steep. I was in 34×25 the whole way up but spun out the 11 going down!”
If you’re new to cycling, gear designations can seem like advanced mathematical theory. When riders start throwing these numbers around, you don’t know what they’re talking about. But gearing, even with the proliferation of choices resulting from 11-speed cassettes and compact cranksets, is quite simple.
Gears can be referred to in several ways, but here’s the simplest method. The designation 39×19 (say “thirty-nine nineteen”), for example, means that the chain is on a 39-tooth chainring (the size of the smaller ring on most “standard” road bikes with 2-chainring, or double, cranksets) and it’s on a 19-tooth cassette cog.
The largest gear on most “standard” road bikes is 53×11 (50×11 for compact cranks). When you see or hear a designation like this, the chainring is always the first number. The cassette is the second.
Lots of roadies still ride the standard 53×39 cranksets, but over the past several years, compact cranksets, typically 50×34, have become extremely popular. So you may well hear “50×11” or “34-28” just as often.
Easy enough? Here’s a twist. You can also calculate gear inches, a throwback to merry old English cycling. The value in doing this is being able to compare different chainring/cog combinations and know which gives you a higher or lower gear. Here’s the gear inch formula:
Teeth on chainring divided by teeth on cassette cog, multiplied by 27
For road bikes, 27 is still used even though the wheel standard has become 700C. If your bike has 26-inch wheels, you would multiply by 26. If it’s a Bike Friday with 20-inch wheels, multiply by 20. And so on.
Here’s an example of how gear inches work. Let’s say you are in 39×15. How does that compare with the 53×21 combination that your friend is using? Even though he’s in the big chainring, you are actually in a slightly larger gear — 70 gear inches compared to 68 (rounded off). Apply the formula just given and you’ll see.
If you calculate gear inches for all the chainring/cog combinations on your bike, you can spot large gaps or wasteful overlaps. You may want to install more usable gears by changing to different cog or chainring sizes.
Coach Fred Matheny is an RBR co-founder who has four decades of road cycling and coaching experience. He has written 14 eBooks and eArticles on cycling training, available in RBR’s eBookstore at Coach Fred Matheny, including the classic Complete Book of Road Bike Training, which includes 4 eBooks comprising 250 pages of timeless, detailed advice and training plans. The Complete Book is one of the many perks of an RBR Premium Membership. Click to read Fred’s full bio.